The invention relates to a semiconductor accelerometer, and more particularly to a semiconductor accelerometer with an additional feature for detecting abnormalities in an acceleration sensing performance due to damage of gage resistors on beams.
In the prior art, typical accelerometers may generally be fabricated by the following steps. A semiconductor substrate is processed to form beams and a mass. The beams are formed thereon with a strain gage comprising four resistors for detecting strain at the beams.
When the mass is subjected to a force caused by acceleration in a vertical direction, the mass is oscillated in the vertical direction. As a result of the oscillation, a strain is caused at the beam portion. Thus, the resistivity of the each gage resistors provided on the beams is varied, after which the variation of the resistivity is detected and retrieved as electrical signals.
FIG. 1 illustrates a block circuit diagram of a semiconductor accelerometer in the prior art. The conventional semiconductor accelerometer 1a includes a strain gage 2 comprising four gage resistors forming a Wheatstone bridge circuit, a power supply circuit 3 for supplying a power to the strain gage 2, an amplifier 4a for amplifying and outputting acceleration detecting signals output from the bridge circuit for detecting a variation of the resistivity of the gage resistors 2. The power supply circuit 3 and the bridge circuit 2 serving as the strain gage are connected in series to one another between a power supply terminal 8 and a ground signal terminal 11. The amplifier 4a amplifies a potential difference between output terminals of the bridge circuit and outputs the acceleration signals to an acceleration signal terminal 10.
FIG. 2A is a plan view illustrating a semiconductor accelerometer chip 16. FIG. 2B is a cross sectional elevation view illustrative of the accelerometer of FIG. 2A. In the typical semiconductor accelerometer chip, the gage resistors 2 are formed on the beams 20. When the chip is accelerated and oscillated in a vertical direction, the mass 19 is subjected to a force caused by the acceleration. As a result of the force, a strain is generated at the beams 20 with which the resistivity of the each gage resistors 2 formed on the beams is varied. Then, the balance loss of the bridge circuit comprising the gage resistors appears. The bridge circuit 2 generates a potential difference between output terminals. The potential difference is likely to be associated with the acceleration. The amplifier 4a amplifies the potential difference and outputs electrical signals (to each of four bonding pads 17 to which terminals of the bridge circuit 2 are respectively connected) which are provided for connection to external devices. The chip is further provided with grooves 18.
The power supply circuit 3 may includes an additional feature for controlling a current to be supplied to the gage resistors 2 according to an ambient temperature thereby enabling compensation of the sensitivity of the gage resistors 2 which depends upon the ambient temperature. While the conventional accelerometer chip 16 illustrated in FIG. 2A includes the bridge circuit comprising the gage resistors 2 only, the chip 16 may further include the power supply circuit 3 and the amplifier 4a by using the semiconductor integration processes.
Since the beam 20 is so formed as to be extremely thin, the beams 20 are frequently accidentally broken and disconnected by overloads. The result of this is that the gage resistor 2 which is provided on the disconnected beam 20 is also broken. Under such circumstances, the accelerometer is no longer able to fulfill the correct detection of the acceleration. It is, however, difficult for the prior art to recognize that the gage resistors are damaged from the output signals only. The reason for this is as follows. The output signals are likely to depend upon the characteristic of the amplifier 4a. As an example, if the bridge circuit loses two gage resistors at the high voltage side connected to the power supply circuit 3, the output terminals of the bridge circuit are commonly taken to ground potential. Then, an offset voltage remains across input terminals of the amplifier 4a and is outputted through the amplifier 4a as output signals.
With respect to the structure, the mass is supported at its opposite ends by extremely thin beams having a thickness of 10 to 20 micrometers. Such structure allows the beams to be accidentally disconnected or broken due to overshock and twist. When this happens, the accelerometer is no longer able to perform a correct sensing of the acceleration, which is considerable. Although this accident should promptly be recognized, the conventional accelerometer has no detecting capability.